PVR Comparisons is a great site for information and (best of all) screenshots of many popular PVR models available today. I'm often asked by people if they should go with their cable company's free or cheap recorder or pony up the dough for a TiVo, and I usually list the pros and cons of the decision, but this site serves as a great information resource for the plethora of choices. They cover all aspects of each unit including capabilities, cost, and features as part of every review. This master feature chart is a good overview of everything out there.
I had no idea what the comcast DVR looked like until I saw this (ack, the colors! the fonts! man, that's an ugly UI). It's an exhaustive resource without equal, and I certainly hope they keep updating it.